Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory, including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), resistance variable memory, and flash memory, among others. Types of resistance variable memory include phase change memory, conductive bridging memory, and resistive random access memory (RRAM), among others.
Memory devices are utilized as non-volatile memory for a wide range of electronic applications in need of high memory densities, high reliability, and data retention without power. Non-volatile memory may be used in, for example, personal computers, portable memory sticks, solid state drives (SSDs), digital cameras, cellular telephones, portable music players such as MP3 players, movie players, and other electronic devices.
Resistance variable memory devices include resistance variable memory cells that store data based on the resistance level of a storage element. The cells can be programmed to a desired state, e.g., corresponding to a resistance level, such as by applying sources of energy, such as positive or negative voltages to the cells for some duration. Some resistance variable memory cells can be programmed to multiple states such that they can represent, e.g., store, more than one bit of data.
The programmed state of a resistance variable memory cell may be determined, e.g., read, for example, by sensing current through the selected resistive memory cell responsive to an applied voltage. The sensed current, which varies based on the resistance level of the memory cell, can indicate the programmed state of the memory cell.
Memory devices are becoming increasingly smaller, with memory cell feature size shrinking. This allows for much higher memory cell device density on chips, and lower cost per memory cell. A memory device, such as a memory array, can have memory cells formed above control circuitry, e.g., decoding circuits, peripheral circuits, etc. When devices were relatively large, one level, e.g., layer, of metal was adequate to provide the metal interconnections of the memory device. In a single level metallization technique, contact can be made to the underlying control circuitry, e.g., silicon devices, by contact holes etched through dielectric materials separating the control circuitry from the conductive material, e.g., metal, used for interconnections. As memory device dimensions have shrunk, multilevel metallization techniques have been used to reduce certain metal dimensions.